The present invention relates generally to compositions and methods for preventing or ameliorating brain tissue inflammation, and more specifically, relates to compositions and methods for modulating leukocyte adhesion to brain endothelial cell receptors.
Leukocytes are white blood cells that travel continuously in the general circulation. At the site of an injury or other inflammatory stimulus, endothelial cells that line blood vessels become activated to express molecules that are adhesive for leukocytes. The mechanics of leukocyte adhesion to endothelial cells involves, in part, the recognition and binding of cell surface receptors on leukocytes to the corresponding cell surface molecules on endothelia. Once bound, the leukocytes migrate across the blood vessel wall to enter the injured site and release chemical mediators to combat infection. For reviews of adhesion receptors of the immune system, see generally, Springer, Nature 346:425-434 (1990), and Osborn, Cell 62:3-6 (1990), both of which are incorporated herein by reference.
Inflammatory brain disorders, such as experimental autoimmune encephalomyelitis (EAE), multiple sclerosis (MS) and meningitis, are examples of central nervous system disorders in which the endothelium/leukocyte adhesion mechanism results in devastating destruction to otherwise healthy brain tissue. Large numbers of leukocytes migrate across the blood brain barrier (BBB) in subjects with these inflammatory diseases. The leukocytes release toxic mediators that cause extensive tissue damage resulting in impaired nerve conduction and paralysis. See generally, Hickey, Psychneuroimmunology II (Academic Press 1990).
In other organ systems, researchers have had some success preventing tissue damage by inhibiting en mass migration of leukocytes across blood vessels. It has been shown that in spite of the initial insult following myocardial ischemia to heart tissue, if leukocyte binding to the endothelium at the damaged site is inhibited, the leukocytes do not enter the tissue and further damage is greatly ameliorated. Vedder et al., Surgery 106:509 (1989). Simpson et al., J. Clin. Invest. 81:624-629 (1988) disclose that the administration of a monoclonal antibody that binds to a leukocyte cell adhesion-promoting glycoprotein (Mol; CD11b/CD18) resulted in reduced injury to heart tissue because fewer neutrophils (a type of leukocyte) entered the heart tissue. Hession et al. in International Patent Publication WO 90/13300, published Nov. 15, 1990, report that antibodies specific for various endothelial adhesion molecules (ELAMS) inhibit the binding of such ELAMS to cultured umbilical vein endothelial cells. Similarly, Wayner in WO 91/03252, published Mar. 21, 1991, describes the ability of monoclonal antibodies to block the adhesion of fibronectin to various hemapoietic cells, including those derived from human spleen, tonsil and peripheral blood.
Brain endothelium, however, is physiologically different from the endothelium associated with other organs, including those described in the above studies. Moreover, the expression of cell adhesion molecules is not predictable and may vary widely in response to different inflammatory stimuli and in different anatomical locations. For example, Tuomanen et al., J. Ex. Med. 170:959-968 (1989), show that antibodies directed against the CD18 family of adhesion-promoting receptors block the migration of leukocytes across the blood brain barrier (BBB) in response to acute inflammatory stimulus of bacterial origin. However, anti-CD18 was shown not to block leukocyte migration to the lung when stimulated by streptococcal infection, T. M. Carlos and J. M. Harb, Immun. Rev. 114:5-28 (1990). Furthermore, antibodies directed against the adhesion molecule VLA-4 are not sufficient by themselves to block monocyte (a type of leukocyte) entry into the inflamed peritoneum. As a consequence of the uniqueness of brain endothelium, compounded by the apparent multiplicity and specificity of the cell adhesion molecules in general, progress in identifying specific cell adhesion molecules and their complementary leukocyte receptors as well as the nature of their interaction has been slow.
One leukocyte cell surface receptor that has received attention from researchers is VLA-4, first identified by Hemler and Takada, EP 330,506, published Aug. 30, 1989. VLA-4 is a member of the .beta.1 integrin family of cell surface receptors, each of which is comprised of two subunits, an .alpha. chain and a .beta. chain. VIA-4 contains an .alpha.4 chain and a .beta.1 chain. There are at least six .beta.1 integrins, all sharing the same .beta.1 chain and each having a distinct .alpha. chain. These six receptors all bind a different complement of the various cell matrix molecules, such as fibronectin, laminin, and collagen. VLA-4, for example, binds to fibronectin. VLA-4 is unique in that it also binds non-matrix molecules that are expressed by endothelial cells. At least one of these non-matrix molecules is called VCAM-1, which is expressed on cytokine-activated human umbilical vein endothelial cells in culture. Distinct epitopes of VLA-4 are responsible for the fibronectin and VCAM-1 binding activities and each activity has been shown to be inhibited independently. Elices et al., Cell 60:577-584 (1990).
One of the monoclonal antibodies used, HP2/1, reacts with the .alpha. chain of VLA-4 and blocks binding to both VCAM-1 and fibronectin. It does not affect the activity of the other members of the .beta.1 integrin family. However, the .alpha. chain of VLA-4 also interacts with a distinct .beta. chain, called 62 7 (formerly called .beta.p). This receptor mediates lymphocyte binding to intestinal lymphoid tissues. For example, HP2/1, reacting with .alpha.4, blocks the activity of the intestinal receptor, i.e., .alpha.4.beta.7 (illustrated in Table 2). The monoclonal antibody, AIIB2, reacts with the .beta.1 chain that is common to all members of the .beta.1 integrins and potentially immunoreacts with the entire family, including the fibronectin and the VCAM-1 binding activities of VLA-4. It would not be expected to inhibit lymphocyte binding to intestinal endothelium, however, since this interaction appears to involve .alpha.4 associated with .beta.7 rather than .beta.1. Prior to the present invention it was not known whether there existed an adhesion molecule on brain endothelial cells complementary to the VLA-4 receptor and, if one existed, whether VLA-4 binding to it could be modulated by these or other reagents.
Due to the unique nature of brain endothelial cells, the unpredictability of leukocyte/endothelial receptor interaction and, prior to the filing of U.S. Ser. No. 07/413,274, the unavailability of an effective in vitro BBB model on which tests could be conducted, little has been learned about the different types of adhesion molecules expressed on brain endothelial cells during brain inflammation, and little has been accomplished to ameliorate the effects of chronic inflammatory brain diseases, such as MS.
Accordingly, a need exists for compositions and methods for modulating the adhesion of leukocytes to brain endothelial cells. There also exists a need for methods of preventing or ameliorating brain inflammation characteristic of diseases such as MS. The present invention satisfies these needs and provides related advantages as well.